This invention relates to an electrostatographic printing machine, and more particularly concerns an apparatus for fusing colored images.
In a typical electrostatographic printing machine, a latent image is recorded on a surface and developed with charged particles. After the latent image is developed, a sheet of support material is positioned closely adjacent thereto so as to receive the particles therefrom. The particles are then permanently affixed to the sheet of support material forming a copy of the original document thereon. Electrographic and electrophotographic printing are differing versions of electrostatographic printing. The process of electrophotographic printing employs a photoconductive member arranged to be charged to a substantially uniform level. The charged photoconductive member is exposed to a light image of an original document. The light image irradiates the charged photoconductive member dissipating the charge in accordance with the intensity of the light transmitted thereto. This records an electrostatic latent image on the photoconductive surface. Electrographic printing differs from electrophotographic printing in that neither a photoconductive member nor a light image of the original document are required to create a latent image on the surface. Both of the foregoing processes generally employ heat settable particles to develop the latent image. The particles are commonly fused to the sheet of support material by the application of heat and pressure thereto.
Various techniques have been developed for applying heat to the particles on the sheet of support material. One technique is to pass the sheet of support material with the powder image thereon through a pair of opposed rollers. In one such system, a heated fuser roll and a non-heated backup roll are employed.
In the most commonly employed type of heated roll fuser, the heated fuser roll has the outer surface thereof covered with a polytetrafluoroethylene commonly known as Teflon to which a release agent such as silicone oil is applied. The Teflon layer, preferably, has a thickness of about several mils.
More recently fuser systems have been utilizing silicone rubber fuser rolls for contacting the toner images to thereby enhance copy quality, that is to say, perceived copy quality.
Bare roll fusers while not commercially accepted have been making inroads, at least in the patent literature. Heretofore, however, no single contact fusing device has been developed which satisfactorily fuses colored toner images. This is because none of the aforementioned fusers can singly fulfill all of the requirements to yield acceptable copy quality.
The requirements for acceptable copy quality are as follows: 1. adequate fix (i.e. toner coalescence and adherence to the paper); 2. adequate fuse level (i.e. toner rendered sufficiently transparent to allow substractive color reproduction to occur); 3. maximum color saturation (i.e. halftone dot spreading at moderate area coverages, 20-90%); 4. minimization of noise in low coverage (5-20%) highlight regions (i.e. dot spreading minimization for background and granularity enhancement) and 5. uniform image gloss independent of pile height (i.e. final image surface must be smooth to minimize de-saturation due to light scattering).
In addition to the failure of known roll fusers to singly satisfy all of the foregoing requirements other systems such as radiant or solvent systems also fall short of being acceptable. For example, in the case of a single step non-image contacting fuser (i.e. radiant or solvent), dot spreading is minimal over most of the input coverages so requirement #3 is not satisfied.
Furthermore, such sytems violate requirement #5. In this regard it can be shown, for examle, that conventional non-contacting systems will inherently deliver a pile height dependent gloss. If the toner has a viscosity of 4.35.times.10.sup.3 poise, for example, it can be shown that the time it takes for an initial toner surface non-uniformity (with characteristic wavelength of 50 microns) to decrease to e.sup.-1 of its initial value depends upon the pile height as follows:
______________________________________ HEIGHT (MICRONS) TIME (SECS.) ______________________________________ 6.4 1.4 5.0 2.1 3.0 7.2 1.0 4.2 ______________________________________
Consequently, in any practical system a single step non-contacting system will deliver an image gloss which is pile height dependent, thus violating requirement #5.
Consider next the case of a single step non-conformable fuser. With conventional toners these systems can satisfy all of the requirements except #4. Dot spreading depends upon the characteristic parameter ##EQU1## where P.sub.o is the pressure on the halftone dot, t is the dwell time and .tau.(T) is a temperature dependent viscosity like parameter. The factor n accounts for the non-Newtonian flow behavior of the toner. For most toner materials n.apprch.0.6. In the non-conformable system, the image P.sub.o can greatly exceed the average nip pressure, p. In the case of a perfectly smooth and rigid paper support, EQU P.sub.o =P/C.sub.A ( 2)
where C.sub.A is the fractional area coverage. Accordingly, single-step non-conformable fusers have their greatest dot-squashing effect in the low coverage, highlight regions. Such systems therefore tend to amplify any non-uniformity in the unfused halftone dot pattern, thus violating requirement #4.
Note, however, that equation (2) indicates that at moderate coverages (30-90%) the image pressure is not drastically different from the nip pressure, so non-conformable systems can operate without excessive noise amplification in this region.
Finally, consider the case of a single step conformable fuser. In this case the image pressure is nearly equal to the nip pressure so the term P.sup.1/n t/.tau.(T) can be made small enough so that minimum dot squashing occurs at the lower coverages while simultaneously achieving dot spreading in the mid coverage range. Single step conformable contact fusers can therefore satisfy requirements #1, #2, #3 and #4, and this has been experimentally verified. However, experimentally it can be shown that the energy (energy delivered to the toner layer) required to achieve a sufficiently high gloss is nearly twice the value needed in the non-conformable bare metal roll fuser. This evidence indicates that the "fusing" step (requirement #2) and the "glossing" step (requirement #5) are independent and are peculiar to the inherent fusing technique. In addition, single step conformable systems violate the second part of requirement #5 in that the fused halftone image is not smooth, since the conformble system can accommodate the micro-structure of the dot shape.
In order to satisfy all of the requirements listed above, this invention suggests a generic fusing system wherein the "fusing" and "glossing" steps are separated.